The invention concerns a process for measuring the isotope ratio, specifically of stable isotopes of chemical substances in a gas to be examined, the process featuring the following steps:
charging a sample cell alternately with a reference gas, and a measuring gas containing the gas to be examined; and PA1 determining the content of isotopes of the substance in the alternately available gases;
and concerns a device for the application of the process.
Such a process is known from chapter 16 of the book titled "Tracers in Metabolic Research--Radioisotopes and Stable Isotope/Mass Spectrometry Methods" by Robert R. Wolfe, Alan R. Liss Inc., N.Y. (1984). A reference gas is on hand in a reference gas cell and a measuring gas in a measuring gas cell. The pressure in these two cells is adapted to one another via a vacuum line that connects them and leads to a pump. A molecular gas flow is passed through a capillary line from each of the cells to a switching valve, the outlet of which latter leads to a mass spectrometer in which the isotopes, due to their different masses, are differently accelerated and spatially separated on different detectors.
Using the mass spectrometer, the measuring gas and the reference gas are in a time sequence cyclically analyzed in succession. This makes it possible to enhance the accuracy of the measurement because all of the test conditions of the mass spectrometer, such as the condition of the source, of the analyzer means etc., can be held constant while the measurement with the measuring gas and the reference gas is conducted.
The greater the number of cyclic measurements performed and the shorter the time spacing between changeovers is selected, the better will be the accuracy of the isotope ratio determined. Therefore, the disadvantage associated with the process is that in the case of longer periods between the changeovers, same test conditions of the following isotope analyzer means cannot be guaranteed.
This is particularly advantageous when using the known process for the infrared-spectroscopic determination of the isotope ratio, since fluctuations of the test conditions, such as the frequency and intensity constancy of the laser, for instance due to the temperature of the laser diode chip, may occur also within short time periods.
From "High resolution infrared diode laser spectroscopy for isotope analysis--Measurement of isotopic carbon monoxide" from Appl. Phys. Lett. 48, page 619 (1986) there is known to determine the isotope ratio of stable isotopes with the use of a tunable infrared diode laser. The prior system uses a temperature-stabilized lead salt diode laser and a two-part sample cell, each with a predetermined light path through the differently long sections of the sample cell.
Usually, the gaseous substance of interest is primarily available in a predetermined natural isotope composition, which in addition to a major isotope features traces of other isotopes whose concentration frequently is lower by a factor in the order of 100. Therefore, the long light path serves to amplify the extinction of the lesser occurring isotope, in order to balance the relative strength of the extinctions of the individual isotopes among one another and to utilize in a similar fashion the measuring sensitivity of the detectors.
The lead salt diode laser impinges alternately on the long and the short arm of the sample cell containing the measuring gas. By shifting the sample cell, the laser beam traverses either the short or the long arm of the sample cell and impinges on a detector. The concentration of the individual isotopes can be calculated from the respective extinction of the laser beam, with the aid of Lambert-Beer's law. The relative error of the isotope distribution so measured is stated to be 2.5 per mill in the prior art and, thus, is by a factor of 25 greater than the corresponding error in the mass spectrometric measurement.
Based on this prior art, the invention has as its underlying problem of providing a process which with a reduced cyclic measuring time features a greater accuracy in determining the relative isotope ratio.